The method of spectral analysis has been known and become established for laboratory use for a long time. Various approaches are possible. Samples which themselves emit radiation within an addressable range may be analyzed directly, so-called “emission spectroscopy”.
If the sample emits no radiation, or if the intensity of the spectral range addressed is not sufficient, additional illumination means may be used. This is usually the case with room temperature. Following interaction with the material of the sample, the change in the incident light is analyzed by means of a suitable system.
The light source may be of various kinds. Often, one uses broadband emission sources whose spectral distribution comes as close as possible to the physical ideal of a so-called black-body emitter, or broadband emission sources which at least come close to same and exhibit high stability. Alternatively, illumination may also be effected by means of a spectrally narrow-band light source. Within this context, several methods are possible.
So-called Raman spectroscopy analyzes the shift in a wavelength, typically of a very narrow-band laser, even though both higher and lower wavelengths are possible.
Fluorescence measurements exploit excitation by means of a wavelength and sense a fluorescent wavelength of a well-defined fluorescent process which deviates therefrom. If a light source is available whose spectral distribution may be adjusted (a so-called tunable light source), said light source may be advantageously used for spectral analytics. In addition, it is possible to provide such an arrangement of a tunable light source by combining a broadband light source and a spectroscopic instrument.
As far as terms are concerned, specialized literature distinguishes between monochromators, i.e. devices which break up incident spectral distribution into its constituents, and spectrometers, i.e. devices which make available the intensity distribution of the incident light in a suitable form. Depending on the image field present at the optical output of such a device, one may then also distinguish between a spectrograph (the image field is corrected in one dimension) or an imaging spectrometer (the image field is corrected in two dimensions). Under certain circumstances, this is very important for specific applications. The solution of a spectroscopic instrument which is described here may be fundamentally used for all design variants. The term “spectroscopic instrument”, or sometimes also spectrometer, will be used as a generic term for all types, including a monochromator.
Conventional technology discloses numerous system approaches capable of analyzing a spectral distribution of electromagnetic radiation.
Previous spectrometers have used a screen on which the spectrum could be displayed and viewed by a user. In addition, the spectra could be captured by using classical photo plates and could be analyzed in terms of quality and quantity. Within this context, both prisms and gratings could be used as the diffractive element. Joseph von Fraunhofer (1787-1826) became famous, among other things, for discovering the sodium lines within the solar spectrum.
However, prism spectrographs are disadvantageous because of the inevitable absorption by the material, which is usually non-ideal, of the prism. What is advantageous are grating spectrometers, which benefit from the diffraction of light at periodic structures in transmission or reflection. During the course of the 20th century, different arrangements were described by Czerny-Turner, Ebert-Fastie and Monk-Gilleson, among others. Particular variants, for example gratings with specific structures (blaze) or the so-called Littrow mounting, exhibit specific advantages. Detailed descriptions are found in specialized literature.
With the advent of electrically readable detectors, the development continued and resulted in a scanning monochromator, wherein the intensity is effected by a photosensitive circuit. Adjustable diffractive elements allow tuning of the system and sensing of an overall spectrum.
The fact that the detectors were developed further into linear or planar arrangements (detector arrays) enabled development of so-called diode array spectrometers. Said specific variant of embodying a grating spectrograph comprises a fixed grating and a detector array or a camera. Of said type, there also exist embodied variants heavily reduced in size. The installation size is limited by the size of the detector.
For the visible spectral range, elements having very small dimensions are available. They are also available at very low price on account of utilization of silicon semiconductor technologies.
For the so-called infrared range, which is important for analyzing organic matter, in particular, silicon detectors so far can only be used for up to about 1100 nm due to the spectral sensitivity of the material. For anything above said value, less widespread material or material combinations may be employed. Specifically for array arrangements, these are very costly and comparatively large. Typical element widths amount to at least 25 μm, but typically 50 μm or more, since the signal noise otherwise becomes problematic.
A spectral range useful for analysis requirements typically includes a spectral width of 900 nm to 1000 nm or more. The resolution should be at least 10 nm to provide reliable evaluation. This results in that the detectors may comprise about 100 elements, so that, therefore, with increments of 2, which are typically used in digital technology, elements of 128 or more may typically be employed. This results in a width of at least 6 mm for the detector and, therefore, in addition to the high cost, also in a limited miniaturization potential. In addition, the optical setup may provide a correspondingly wide image field, which involves additional expenditure.
In parallel with developing diffractive spectroscopic instruments, systems based on interferometers have been implemented. Important representatives are Fabry Perrot filters and Fourier transformation (FT) spectrometers. Within this context, the spectral characteristic of an interferometer is changed and/or tuned, and the intensity distribution sensed at the same time is evaluated. The spectral intensity distribution is calculated by means of suitable transformation of the data. Such approaches have existed for a long time. Because of the sensitivity of the interferometer to vibrations, corresponding measures may be used. Current developments of MEMS-based Fabry Perrot filters and silicon-etched FT spectrometers are promising approaches regarding highly miniaturized systems within the field of infrared spectral analysis applications.
One important step toward implementing low-cost NIR spectrometers was to develop the “scanning grating spectrometer”. EP 1 474 665 and EP 1 474 666 describe MEMS-based approaches (MEMS=micro-electromechanical system) which utilize a movable element and therefore make do with one single detector. In addition to the advantage in terms of cost, the dimensions of the system may be considerably reduced. The systems are very robust and may also be employed outside laboratory premises. Miniaturization is essentially limited by the useful adjustment of the components.
A further development of this approach was implemented by means of an advantageous manufacturing variant described in DE 10 2008 019 600 and has been referred to as MEMS hybrid spectrometer. Here, further miniaturization is achieved by two essential improvements. For one thing, more functional elements are integrated into the MEMS component. As a result, adjustment of the grating and of the slits may be effected by photolithography of process technology with a precision level that is much higher than that of any component adjustment. For another thing, the spectrometer is implemented as a stack of substrates, so that it will be possible, in principle, to build a large number of systems as a compound, to simplify adjustment and to subsequently separate the systems.
The properties of this approach to a system have been examined in detail and described in specialized literature [1]. The nature of the approach described, wherein the grating is located, in its non-deflected idle position, within the chip plane along with the two slits, has resulted in a new mathematical description [2]. The symmetry of the approach, which is due to the principle employed, i.e. the fact that the grating will usually oscillate to the same extent in both angular directions starting from the idle position, involves deviating from the classical approach of a Czerny-Turner spectrometer in the first-order diffraction since otherwise the same spectral range will be swept twice for positive and negative diffraction angles. This problem has been solved by using the first negative diffraction order. Within this context, the W-shaped optical path of the Czerny-Turner spectrometer is folded, and the entrance slit and the exit slits are shifted to the same side of the grating. One has succeeded in implementing a spectrometer having a design size of only 18×16×10 mm3.
It has been shown [3] that the design height may be reduced to as little as 6 mm or less, whereby integration into a mobile phone becomes possible. However, utilization of the folded optical path also entails a problem. Instead of the spherical on-axis mirrors that are commonly used with Czerny-Turner, biconical off-axis mirrors may be employed in order to achieve acceptable imaging performance. They are complex to manufacture, and with the technologies currently available, production in large numbers is possible only to a limited extent and/or not at sufficiently low cost.
A specific embodiment variant of the spectrometer with a fixed grating was presented in the last few years. Here, the necessity of a detector row is replaced by utilizing a spatial light modulator (digital light processor, DLP). A spatial light modulator, here the known DLP chip by Texas Instruments from the projector (“beamer”), is positioned within the image plane of the spectrograph having a fixed grating, and the spectrum is suitably imaged onto an individual detector.
During operation, a mirror is selected and positioned such that its corresponding spectral intensities of the wavelength interval impinge upon the (individual) detector, and all other wavelength ranges are masked out. Said promising approach is being employed in first products. The miniaturization potential is limited by the DLP design size, and the costs of the DLP are comparatively low as compared to current near-infrared spectrometers but are a limiting factor for utilization in mobile phones, as is its size.
The above-described methods of spectral analysis are currently becoming established from laboratory methods to standard methods for application in the field. In addition to portable systems for professional users, first devices for mass applications have been put on the market and are about to be employed in mobile phones.
For reliable spectral analysis, laboratory methods have been known and become established. In recent years, there has been a lot of investment in miniaturizing the components that may be employed for mobile use. One promising approach is MEMS-based spectral analytics, which exhibits advantages as compared to systems having fixed gratings in particular within the so-called near-infrared (NIR) spectral range, i.e. electromagnetic radiation within a wavelength interval from 780 nm to 2500 nm. With regard to the design size, considerable progress has been achieved [3]. Sufficient miniaturization seems feasible.
Many applications of spectral analysis additionally benefit from a large measuring range, which might start with ultraviolet (UV), include visible light (VIS) and reach as far as infrared (IR). The larger the numbers of pieces envisaged, the lower the implementation expenditure may turn out. Nevertheless, the user expects high resolution, high stability and reliability for his/her portable system, which should be as compact as possible.
The question concerning the spectral range may be solved, e.g., via the modulation range of a movable grating or via the width of a detector array in connection with a fixed grating. In addition to physical limits, for example the maximum deflectability of a miniaturized grating, economic aspects and the installation size achievable are also important. The system approaches disclosed in conventional technology impose limits in this respect.
Miniaturization is limited, in particular, by the useful adjustment of the individual components. To solve this problem, DE 100 61 765 A1 proposes integrating slit diaphragms into a torsion element. The slit diaphragms are arranged along the tilting, or torsional, axis of the torsion element, which renders production of the torsion element more difficult and, therefore, more expensive, however. The grating arranged on the torsion element additionally involves a high degree of deflection of the torsion element so as to select the individual wavelength ranges.
It would therefore be desirable to provide an improved spectroscopic instrument which solves the above-mentioned problems of conventional technology. In particular, it would be desirable to provide a spectral analysis system capable of addressing a wide spectral range given a high degree of miniaturization while being able to be implemented advantageously both with regard to installation size and manufacturing expenditure. In particular, the modulation range of the active component (e.g., MEMS mirror) should be as small as possible, and the technology used should be implemented in as simple a manner as possible so as to be able to be manufactured in mass production while being low in cost at the same time.